The weighted inequality for cross-intersecting pairs with profile-dependent skewness

Let (A1,B1),,(Am,Bm)(A_1,B_1),\dots,(A_m,B_m) be pairs of finite non-empty subsets of N\mathbb{N}. Write ai=Aia_i=|A_i| and bi=Bib_i=|B_i| for 1im1\leq i\leq m. Assume that

AiBi=(1im),A_i\cap B_i=\emptyset\quad (1\leq i\leq m), AiBj(1i<jm),A_i\cap B_j\ne\emptyset\quad (1\leq i<j\leq m),

and AiBjA_i\cap B_j\ne\emptyset whenever AiAj|A_i|\ne|A_j| or BiBj|B_i|\ne|B_j|. Weighted cross-intersection conjecture. Then

i=1m1(ai+biai)1.\sum_{i=1}^m\frac{1}{\binom{a_i+b_i}{a_i}}\leq 1.

This asks whether full cross-intersection for pairs with distinct profiles, together with only the stated ordered condition for pairs of the same profile, still implies the Bollobás-type weighted bound. The surrounding examples show that relaxing the ordering condition can allow weighted sums greater than 11, so the validity of this particular relaxation remains unresolved.

Sources & referencesView supporting material

Primary source

Alex Scott and Elizabeth Wilmer, “Combinatorics in the exterior algebra and the Bollobás Two Families Theorem”, arXiv:1907.06019 (2020).

Progress summary

Never refreshed

Nothing recorded yet. Refresh searches the literature and the public web for attempts on this problem, and writes the first summary here.

Solutions 0

No solutions have been posted yet.